The human baby is the least developed of any mammal. There are various theories why the human baby is born at such an early stage of development (all humans, like marsupials, are essentially born 3 months before they're ready); the naive guess is because later birth would not fit in the mother, but modern theories are different (metabolic or developmental).
A baby is pretty crap as a human, but it's pretty good as an infant marsupial. It wants to just lie on the mother's chest and sleep and eat. Once you think of it as a marsupial lots of other things are just obvious, like it needs low light and not very much stimulation. If you try to make it do anything other than what a marsupial wants (like sleep without skin contact) it gets upset, of course. It really struck me when watching our baby do a sort of proto-crawl (really just a random moving of the limbs) and wiggling around trying to get from the chest to the nipple; that evolved proto-crawl is useless to get along the land to the mother, the only thing it can do is move the baby around inside the marsupial sack.
The Karp method is at its core one sentence of information - "babies are soothed by recreating a womb-like environment". But it's even more accurate to say that what you want to do is create a marsupial-pouch-like environment (eg. you aren't putting the baby in total darkness and immersing it in fluid, nor are you feeding it through a tube).
As is often the case with childrearing, ancient man does a better job than modern man. I think the ideal way to deal with a newborn is just to tuck it in mama's shirt and leave it there. It sleeps on the chest, suckles whenever it wants, and bonds to mama in a calm, sheltered, warm environment. Being tool-using animals, we make our missing marsupial pouch from some cloth. The modern baby carrier things are okay (especially the ones that have the baby facing you on the chest), but they're wrong in a major aspect - they're worn outside the clothing, the baby should really be right on your skin.
It's really incredible how badly we fucked up childbirth and rearing in the western world in the last 100-200 years. Granted it was for good reason - babies and moms used to die all the time. I don't romanticize the old days of home birth with all its lack of sanitation and high mortality rates, the way some of the more foolish hippy homebirth types do. Birth was a major medical problem that needed to be fixed, it's just that we fixed it in massively wrong ways.
I'm trying as much as possible to not read any baby books or crap on the internet. I don't want to be influenced by all the trends and crap advice out there. I want to just observe my baby and do what seems obviously right to me. So for example, I was aware of "attachment parenting" or "aware parenting" or whatever the fuck the cult is called now, but I hadn't made up my mind that we would try that. But once we had the baby and I just looked at it, it was totally obvious that that's what you should do.
If you just pay attention to your baby, it's so clearly trying to tell you "I'm hungry" or "I'm sleepy" or "the light is too bright" or whatever. The only time that a healthy baby cries is when you have ignored its communication for quite a long time (5-10 minutes) and it's gotten frustrated and fed up; a baby crying is it saying "you are neglecting me, please please help me I can't help myself, you fucker, I already asked you nicely". (of course it's not the same in unhealthy babies; and we have some issues like acid reflux and/or gas that lead to some amount of crying inevitably; it's not that all crying is necessarily bad parenting, but there are enough cases of crying that are a result of not listening to the earlier gentle communication that it just seems obvious that you should take care of the baby's needs before it gets to the point of crying, if possible). You've created this helpless little creature, and it gets hungry or uncomfortable and is begging for your help, and to ignore it for so long that it has to yell at you is pretty monstrous.
It's crazy to me that people for so many years thought it was right to just let a baby cry, that it was even good for it to work its lungs or develop independence or not get spoiled. Society gets so stuck in these group-thinks; of course we're all just in a new one now, and it's impossible for me to ever know if I am actually thinking clearly based on what I see, or if I have been subconsciously brainwashed by my awareness of the societal group-think.
(Society considered harmful) (only fools think that they could ever have an independent idea, or any taste or opinion that is actually their own).